The basic idea of a regenerative cycling process to prevent blackening of the envelope of an incandescent lamp was disclosed in U.S. Pat. No. 2,883,571 -- Fridrich et al., which used iodine as the regenerative agent. The concept has since been extended to bromine and chlorine and the former has been used extensively in commercially produced lamps. In such lamps the bulb contains, in addition to an inert filling gas, a reactive transport gas formed by the halogen component which reacts with tungsten evaporated from the filament and deposited on the envelope wall. The transport gas forms a volatile compound with the tungsten which breaks down in the vicinity of the hot filament to redeposit tungsten on the filament. As a result, the bulb wall remains free of blackening and the emitted lumens per watt remain substantially constant til the end of life. However redeposition of tungsten on the filament is not uniform and life ends when the filament burns through in one place.
Correct and satisfactory operation of a halogen regenerative cycle in an incandescent lamp requires that the dimensions of the lamp be chosen so that during operation the temperature of the bulb wall will not permit excessive condensation of tungsten oxyhalides at the wall. Generally a tubular envelope is used with the filament lying on axis, the distance from filament to bulb wall being chosen so that during operation the bulb wall temperature is everywhere above the required minimum. Of course the same lamp operated within an outer jacket will encounter higher temperature conditions than when burnt in open air, that is without a jacket.
The regenerative halogen cycle can be disturbed by the presence within the lamp of a metal, whether present merely as an impurity or deliberately introduced, capable of reacting with the halogen and forming a nonvolatile compound therewith in the lamp because this results in the halogen being withdrawn from the cycle. However the results can also be bad if volatile compounds of the metal are formed, particularly if a transport cycle is set up that removes the metal from some critical place and deposits it elsewhere. For instance if the filament supports are made of such metal and attacked, they can be rapidly cut through and the lamp destroyed.
A problem of the foregoing kind arises in a tungsten halogen lamp containing bromine as the carrier gas and using inner lamp parts of molybdenum. The molybdenum supporting wires may be corroded by the carrier gas until the filament loses its support and sags. Such attacks may be prevented by coating the molybdenum with a noble metal such as platinum but that solution is too expensive to be acceptable. In U.S. Pat. No. 3,538,373 -- Van Der Linden et al. it is proposed to prevent attack of the molybdenum by coating it with a thin film of carbon. Such a solution may be acceptable for relatively short lived lamps, for instance photographic projection lamps having a life expectancy of not over 100 hours, but it is not practical for long lived lamps.